In a conventional SRAM, a high resistance drain junction resistor must be formed by a multiplicity of processing steps using polycrystaline silicon. It is not only a complicated process, but also a very difficult process for controlling the exact magnitude of resistance obtained.
The structure of a conventional SRAM 10 is shown in FIG. 1, wherein a static memory cell is constructed with four-transistors. The gate junctions and the drain junctions are connected together in a corresponding pair of transistors 12 and 14. Each of the drain junctions of transistors 12 and 14 is connected in series with a high resistance drain junction resistor 16 and 18. Word line WL and WL' are connected to the control transistors 20 and 22 to form a static memory cell. During the manufacturing process of the drain junction resistors 16 and 18 of the memory cell, polycrystaline silicon material is used as the poly load for its high resistance and space-saving features. In order to form a resistor by the polycrystaline silicon process, at least four processing steps are required, deposition step, a poly 2 implantation step and a poly 2 etch step. These processing steps leads to a complicated and difficult to control manufacturing process. Furthermore, the process frequently produces a resistance that easily drifts away from its desired value and the process is difficult to precisely control. It is therefore desirable to provide an improved method to overcome these difficulties.
It is therefore an object of tile present invention to provide a method of forming a resistor in a four-transistor SRAM that does not have the shortcomings of the prior art methods.
It is another object of the present invention to provide an improved method of forming a resistor in a four-transistor SRAM that does not require multiple processing steps.
It is a further object of the present invention to provide an improved method to form a resistor structure in a four-transistor SRAM that does not require the formation of a drain junction resistor by polycrystaline silicon.
It is yet another object of the present invention to provide an improved method of forming a resistor structure in a four-transistor SRAM that only requires a photoresist process for implantation and an implantation step to form amorphous silicon in the surface layer at the drain junction.
It is another further object of the present invention to provide an improved method of forming a resistor structure in a four-transistor SRAM by implanting silicon or argon into the surface layer of the drain junction such that a high resistance drain junction resistor is formed between the drain junction and the metal contact region.